1. Field of the Invention
This invention belongs to the field of processes for providing thermoplastic resins in fine powder form and, more particularly, to such processes in which the powders are obtained from aqueous dispersions of resin.
2. Description of the Prior Art
Thermoplastic resins in finely-divided form have found use in a number of applications where it is either impossible or inconvenient to utilize the more conventional cube or pellet forms. For example, powdered organic polymeric thermoplastic resins in dry form have been used to coat articles by dip coating in either a static or fluidized bed, by powder coating wherein the powder is applied by spraying or dusting, and by flame spraying. In dispersed form, thermoplastic resin powders have been applied as coatings by roller coating, spray coating, slush coating, and dip coating to substrates such as metal, paper, paperboard, and the like. These powders have also been widely employed in conventional powder molding techniques. Other applications of these powders include paper pulp additives; mold release agents for rubber; additives to waxes, paints, and polishes; binders for non-woven fabrics; dry-blendable impact modifier powders; and so on.
It is well known that high molecular weight thermoplastic resins, for example, polyethylene and ethylene copolymers, may be converted to dispersions of spherically shaped particles which are substantially devoid of particles greater than 25 microns in diameter and in which the number average particle diameter is less than about 10 microns. Thus, McClain, U.S. Pat. No. 3,422,049, teaches that such dispersions of finely divided particles may readily be prepared by agitating the molten resin in admixture with water at elevated temperatures and at autogeneous pressure, in the presence of certain dispersing agents which are particular block copolymers of ethylene oxide and propylene oxide. Such dispersing agents are available commercially under the trade name Pluronic (BASF-Wyandotte Corp.). Unless the thermoplastic resin is a soft, tacky material that is prone to coalesce under ambient conditions, the nonagglomerated spherical particles can be recovered as powders by cooling the dispersions below about 100.degree. C. and collecting the suspended material by filtration or centrifugation.
U.S. Pat. No. 3,418,265 teaches that the particle size of such thermoplastic resin dispersions can be reduced still further, to the submicron level, while retaining the unique spherical particle shape by including in the dispersion process a volatile, inert, water-insoluble organic liquid that is soluble in the thermoplastic resin in an amount between 0.5 to 20 parts per 100 parts of the resin, whereupon a stable, aqueous, film-forming latex is ultimately obtained as the final product. Alternatively, U.S. Pat. No. 3,522,036 teaches that stable, film-forming aqueous latices of high molecular weight polyethylene can also be formed by including a liquid vinyl monomer such as styrene in the dispersion process.
Although the foregoing dispersion procedures are conveniently operated as batch processes, it is also known to produce such finely divided powders in a sequential, continuous process. See, e.g., U.S. Pat. No. 3,432,483.
U.S. Pat. No. 3,586,654 teaches that it is further possible to conduct the dispersion process in such a way that the polymer particles may be further transformed into spherical particles of controlled average size and size distributions which are the same, larger or smaller than the starting particles. If desired, the dispersion process can be modified in such a manner as to produce spherical foamed particles (U.S. Pat. No. 3,472,801), or to incorporate within the particles certain colorants (U.S. Pat. No. 3,449,291) and pigments (U.S. Pat. No. 3,674,736). In addition, it has been found that various substrates can be coated by applying the above described dispersions of polyolefin fine powders in an inert carrier, heating to evaporate the carrier, and fusing the polyolefin to the substrate (U.S. Pat. No. 3,432,339). Further, U.S. Pat. No. 3,669,922 teaches a process for preparing colored polymer powders having controlled charge and printing characteristics of value as toners in electrostatic printing.
In the above cited art, the dispersing agents, as block copolymers of ethylene oxide and propylene oxide with molecular weights of about 3,500 to as much as 16,250, belong to the class of oligomeric or low molecular weight polymeric dispersing agents, as opposed to the more familiar, conventional dispersing agents such as, for example, fatty acid soaps, sodium lauryl sulfate, etc. For many years it appeared that the aforesaid block copolymers of ethylene oxide and propylene oxide were the only substances capable of directly dispersing high molecular weight thermoplastic resins, especially olefin polymers, in water at elevated temperatures.
Recently, however, it has been shown that certain soaps of the highest acids available commercially, e.g., lithium behenate, are by themselves also capable of dispersing polyethylene (U.S. Pat. No. 4,148,768). Alkali metal soaps of fatty acids such as stearic acid, oleic acid, palmitic acid, myristic acid, etc. are at best only marginally effective in dispersing polyethylene and are quite ineffective in producing polyethylene dispersions at the high resin concentrations and high production rates attainable with the ethylene oxide-propylene oxide copolymer dispersants disclosed above. The alkali metal soaps of the said fatty acids can, however, be made to disperse polyethylene and ethylene-vinyl acetate copolymers containing up to about 15 weight percent of vinyl acetate, at rates and at solids levels comparable to those achievable wit the Pluronics, if said soaps are produced from the fatty acid and alkali in situ during the dispersion process and in the presence of a water-soluble neutral salt; e.g., sodium chloride, sodium sulfate (U.S. Pat. No. 4,148,766). When ethylene-vinyl acetate copolymers containing about 15 to 35 weight percent of vinyl acetate are dispersed, however, a preformed alkali metal fatty acid soap, in the presence of a water-soluble neutral salt, may then be used as the dispersant system (U.S. Pat. No. 4,151,133), and if the ethylene-vinyl acetate copolymer contains above about 35 weight percent of vinyl acetate, the soap alone is a satisfactory dispersing agent, optionally in the presence of a water-soluble neutral salt to control particle size (U.S. Pat. No. 4,150,003).
As pointed out above, block copolymers of ethylene oxide and propylene oxide as dispersants produce spherical particles with a number average particle diameter less than about 10 microns. In general, particle diameter is affected very little by process variables. By contrast, however, the aforesaid newly described soap-salt dispersant systems produce larger spherical particles, with few particles having diameters below 20 microns being observed. Particle size in the soap-salt dispersion system can be varied to some degree by varying the nature of the soap and especially the concentration of the water-soluble neutral salt. Increasing the soap concentration beyond some threshold value has little effect on particle size. Adding alkali beyond the point of neutralization of the fatty acid favors binodal particle size distributions.
Truly high molecular weight dispersing agents have recently been employed to disperse olefinic polymers (U.S. Pat. No. 4,174,335), but the process of that patent is clearly directed to the production of stable aqueous dispersions, and not to the dispersion of such resins into particle size ranges that would lend themselves to recovery as dry powders. Thus, according to U.S. Pat. No. 4,174,335, stable aqueous dispersions are produced by mixing a molten blend of (a) a carboxyl-free olefin resin and (b) a carboxyl-containing olefin resin with an aqueous medium in the presence of at least 0.2 chemical equivalent, to the carboxyl groups, of a base. Typically, aqueous dispersions stable up to seven days at room temperature are produced, the ultrafine particles of which are spherical and no more than 3 microns in diameter. Larger particles were obtained only when the process was operated at too low a dispersion temperature (120.degree. C.) or when less than 0.2 equivalent of base per equivalent carboxyl groups was used, or when the acid number of the carboxyl-contaning olefin resin was less than 20. As explained more fully hereinafter, the process of U.S. Pat. No. 4,174,335 is totally unsuited to making olefin polymer powders of the previously cited art, and especially of the present invention, because the rates of dispersion are exceedingly slow. Moreover, the ultrafine particles produced are too fine for easy isolation, handling and application.